1. Field of the Invention
The present invention relates to a light intensity deterioration detecting circuit for detecting deterioration of the light intensity of a light emitting means such as a light emitter.
2. Description of the Background Art
FIG. 13 is a circuit diagram of a conventional light intensity deterioration detecting circuit. As shown in FIG. 13, the anode of a laser diode 1 serving as a light emitter is connected to a power supply 13, and the cathode of a photodiode 2 serving as a photodetector is connected to the power supply 13. The cathode of the laser diode 1 is connected to a ground terminal 15 through a reference current source 4 and is connected to the output of a current amplifier circuit 3. The anode of the photodiode 2 is connected to a first end of a resistor 9.
The input of the current amplifier circuit 3 is connected to a second end of the resistor 9 and to the collector of an NPN bipolar transistor 6. The NPN bipolar transistor 6 has an emitter connected to the ground terminal 15 through a reference current source 5, and a base connected to the ground terminal 15 through a reference voltage source 7.
The above-mentioned laser diode 1, photodiode 2, current amplifier circuit 3, reference current source 4, reference current source 5, NPN bipolar transistor 6, reference voltage source 7, and resistor 9 form an APC (Automatic Power Control) circuit.
A comparator 11 compares a comparison voltage V91 provided at the first end of the resistor 9 and a comparison voltage V92 provided at the second end thereof to output the comparison result to an output terminal 12.
Operation of the light intensity deterioration detecting circuit shown in FIG. 13 will be described below.
A drive current I.sub.LD flows in the laser diode 1 which then emits light having an intensity based on the drive current I.sub.LD. The amount of the drive current I.sub.LD is determined by subtracting an output current I.sub.OUT of the current amplifier circuit 3 from a reference current I4 of the reference current source 4.
The light emitted from the laser diode 1 is received by the photodiode 2, and a detection current I.sub.PD flows in the resistor 9 on the basis of the intensity of the received light.
A reference potential difference .DELTA.VR for the comparator 11 is previously set, and the comparator 11 compares a potential difference .DELTA.V12 between the comparison voltage V91 and the comparison voltage V92 with the reference potential difference .DELTA.VR to output the comparison result to the output terminal 12.
When a current to light output characteristic of the laser diode 1 is satisfactory, the detection current I.sub.PD increases and the potential difference .DELTA.V12 between the comparison voltages V91 and V92 is greater than the reference potential difference .DELTA.VR. When the current to light output characteristic is deteriorated, the detection voltage I.sub.PD decreases and the potential difference .DELTA.V12 is less than the reference potential difference .DELTA.VR.
Thus, the comparator 11 detects the potential difference .DELTA.V12 between the comparison voltages V91 and V92, compares the potential difference .DELTA.V12 with the reference potential difference .DELTA.VR, and outputs the comparison result, thereby to detect deterioration of the current to light output characteristic of the laser diode 1.
The conventional light intensity deterioration detecting circuit as above constructed detects the voltage across the resistor 9 to detect deterioration of the current to light output characteristic of the light emitter.
This causes the absolute accuracy errors of the resistance of the resistor 9 to be directly reflected as errors of the judgement reference of the comparator 11. For example, .+-.20% resistance variation of the resistor 9 when fabricated results in 20% variation in potential difference .DELTA.V12 across the resistor 9 if the detection current I.sub.PD is the same.
For this reason, it has been difficult to correctly judge the deterioration of the current to light output characteristic of the light emitter. To solve the problem, it has been necessary to provide an offset control for correcting the judgement reference errors of the comparator 11 resulting from the resistance errors of the resistor 9.